North American bees are disappearing at a rapid rate, signaling a dire threat to the production of countless food sources. The Guardian reports that four common species of U.S. bumble bees have declined 96 percent in recent decades, and scientists allege that disease and inbreeding are responsible.

Honey bees have long been known to be in decline, suffering from the enigmatic colony collapse disorder, and the latest research on U.S. bumble bees only exacerbates concerns over future food production, as bees are responsible for pollinating 90 percent of the world’s commercial plants, from fruits and vegetables to coffee and cotton.

While a correlation between the Nosema bombi fungus infection and the declining bumble bee populations was discovered, the culprit isn’t clearly defined. One of the study’s researchers told LiveScience that the data doesn’t necessarily verify that the disease is driving the decline, and other factors — like reduced adaptability to environmental changes as a result of inbreeding — are likely at play.